The problem is that Airbnb doesn’t know if a guest has checked in or not. We all know there are some scammer, crappy hosts out there, so a guest could arrive, but find the place to be a bait and switch, or filthy, etc. and turn around and leave.
So it could just end up being the guest’s word against the host’s that they actually showed up. I imagine that was Airbnb’s thinking when allowing reviews within a day of check in, but as usual, their thinking misses the mark. Reviews shouldn’t be prompted if guest cancels before the stated check-in time.